Central collisions of heavy nuclei at cm. kinetic energies of a few hundred MeV per nucleon produce fireballs of hot, dense nuclear matter. Each fireball explodes, producing a blast wave of nucleons and pions. Several features of the observed cross sections for pions and protons from Ne on Na F at 0.8 GeV/nucleon (lab) are explained by the blast wave, but contradict earlier, purely thermal models. The available energy is equally divided between translational energy of the blast, and thermal motion of the paritlces in the exploding matter.

We consider the conditions under which nuclei will fragment into smaller pieces. We argue that fragmentation will occur when the nuclear matter has expanded adiabatically to the onset of hydrodynamic instability, ~P/b VIs = 0. We discuss the conditions of initial heating and/or compression which lead to fragmentation, and argue that the resulting fragmented matter will be composed mainly of alpha-particles and nucleons.

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